The industry demand for increasing power density and lowering the height of the power converters in power conversion area imposed the use of planar magnetic inductors and planar transformers. One approach of such planar element which can be an inductor or a transformer with energy storage capability includes two E magnetic cores 18,20 which surrounds a printed circuit board 6 embedding the windings made of plated copper 8. To be able to store energy and avoid the saturation of the magnetic material of the cores 18 and 20 a gap 28 is provided on the center leg area. During operation when a current is injected in the windings 8, the magnetic field produced by the windings will close through the magnetic material as depicted by the magnetic field lines 22, 24. The magnetic field lines are going through the outer legs of the magnetic core, 122, and 124, and through the center leg 120. Around the gap area the magnetic field lines are spread outside of the center leg due to the low permeability of the material placed in the gap, which is usually air. Some of these lines cut into the windings 8 as depicted in FIG. 1A. The component of the magnetic field perpendicular on the planar windings 8, Hv 128 will induce eddy current into the winding. The eddy current developed in the winding will create a magnetic field which will oppose the component of the magnetic field 30, perpendicular on the winding 128. The eddy current developed in the planar winding 8, will lead to additional power dissipation reducing the efficiency of the magnetic element and will create temperature rise in the planar winding. The goal for a magnetic storage element is to increase the efficiency with which the magnetic field is stored and further retrieved. The efficiency of a magnetic storage element is defined by the ratio between the energy retrieved form the magnetic element and the sum of the energy which is required to store the magnetic field and the energy stored in the magnetic element.
In order to minimize the losses due to the eddy current induced by the fringe magnetic field around the gap 28, the fringe magnetic field 30, shall have a minimum component vertical to the planar winding. This means that the magnetic field, which links the winding, shall be parallel with the planer winding.
An article entitled “Integrated Planar Inductor Scheme for Multi-module Interleaved Quasi-Square-Wave (QSW) DC/DC Converter”,PESC 1999 Proceedings, pp.759-763, by W. Chen, P. C. Lee, X. Zhou and P. Xu proposes a two air gap arrangements made on the top and bottom section instead of the vertical leg. Since the magnetic field strength distributions in the winding window along the horizontal direction are more evenly distributed and the fringing fluxes near the air gaps are basically in parallel with the copper plated windings the AC losses are lower. They claim a 42.3% AC losses decrease for their approach.
Another articles aimed at reducing the negative impact of the magnetic core gap is entitled “Reduction of Eddy Current by FPC-Foil Windings in Power Transformers” PCIM'97 Proceedings, pp 315-322, by Markus Heckmann and Jurgen Hess, Hong Kong Conference held Oct. 14-17, 1997. In this article several FPC (Ferrite Polymer Composite) foils were wound around the discrete gap to minimize the fringe magnetic field cutting into the winding.
Another prior art concept depicted in FIG. 2 was presented by Bruce Carsten at PCIM'96 seminar held in Nuremberg, Germany. In his seminar Bruce Carsten sugest to combination of a E magnetic cosre using high permeability magnetic material and and I section made of a distributed air gap material.
Another approach depicted in FIG. 2 uses an E+ I cores 30 and 32 structure where the I core 32 can be made of a material, which can store a significant amount of energy. Such material can be powder iron material, cool-mu material or even ferrite material with low magnetic permeability. The powder Iron material and cool-mu material, have a distributed gap. This means that the energy is stored in the material itself in the small gap uniform distributed in its structure. In this patent we will refer to such a material as magnetic core with energy storage capability. As is presented in FIG. 2 the magnetic field lines generated by the current flowing through the planar winding 8, will link the center leg 120 and the outer legs 122 and 124 by the magnetic field lines 38 and 36. The magnetic field lines will also link the distributed storage element 32. Though some of the magnetic field lines will escape the I section 32, as is 34 and 132, the perpendicular component on the planar winding 8, of these magnetic field lines is greater reduced.
The magnetic field lines which escape the magnetic core structure 130, can induce currents into the circuits near by, leading to additional electromagnetic noise. The goal of the magnetic storage elements is to contain most of the magnetic field lines inside its structure to reduce the radiation. The noise induced by the magnetic field lines outside of its structure will negatively impact the EMI compliance imposed by the regulatory agencies. Another drawback of this concept is the fact that the core losses in the iron powder material are generally higher than the ones in the ferrite material, especially at higher frequency.